PHYSICS 535 



the luminosity due to 7-rays had a smaller effect in masking the 

 a-ray scintillations which were to be counted. Using the usual 

 device for preventing the /3-particles striking the screen, that is 

 deflecting them away by a magnetic field, it was possible to 

 count the scintillations with the screen only 2-5 cms. away from 

 a source of radium C, equivalent in 7-ray activity to 20 mgms. of 

 radium. So concordant were the results that observations 

 taken after an interval of six months were found to be in good 

 agreement with those taken previously. A further advantage 

 ensued from the good fortune of a gift of some radio-thorium 

 of unusual activity. With this, deposits of thorium C on a 

 nickel disk were obtained which ejected a-particles with a range 

 of 8-6 cms., decidedly a more vigorous projectile than even the 

 a-particle from radium C with its range of 7 cms. 



With these appliances it has been decided beyond doubt 

 that, whereas the H atoms due to recoil when a-ray particles 

 from radium C pass through hydrogen have a maximum range 

 of 29 cms. in air, i.e. about four times the range of the a-particles 

 themselves, the particles from nitrogen bombarded by these 

 same a-particles have a range of 40 cms. in air. This definitely 

 shows that these particles are not due to the presence of free 

 hydrogen or hydrogen in combination as a contamination. 

 The bending of these particles by a magnetic field agrees with 

 the bending which can be theoretically calculated for particles 

 which have unit charge and the mass of an H atom and move 

 with a velocity corresponding to this range of 40 cms., using 

 Bragg 's rule connecting speed and range. 



It has further been discovered that in addition to nitrogen, 

 particles whose ranges vary from 40 to 90 cms. of air are ejected 

 from boron, fluorine, sodium, aluminium, and phosphorus when 

 these are exposed to intense a-rays. Out of the elements 

 between lithium and sulphur which have been examined the six 

 mentioned above show the effect, the others do not. Sonie 

 other elements of higher atomic weight than sulphur, viz. 

 chlorine, magnesium, calcium, titanium, manganese, iron, 

 copper, tin, silver, and gold, have also been examined without 

 result ; i.e. no particles whose length of range clearly decided 

 that they were not due to hydrogen contamination were observed. 



Of the substances observed aluminium gave particles of a 

 very considerable range, viz. 90 cms., phosphorus coming next 

 with 65 cms. ; boron gives particles with a range about 45 cms., 

 while fluorine and sodium are much the same as nitrogen. 



A notable feature of the particles from aluminium was the fact 

 that the direction of their escape was largely independent of the 

 direction of the bombarding stream of a-particles ; in fact nearly 

 as many were expelled in the backward as in the forward direc- 

 tion ; but the range of the particles shot back was found to be 



